Gas turbine power generation systems that utilize fuels such as coal or low grade oils, use a gasifier to convert the raw fuel to gaseous fuels suitable for the gas turbine. The gasifier unit requires oxygen for the fuel conversion. The required oxygen is supplied from an air separation unit that is advantageously integrated with the gas turbine, as for example shown in U.S. Pat. No. 4,224,045 to Olszewski et al. That patent discloses a system for low temperature fractionation of air which employs a higher pressure rectification column having its upper end in heat exchange communication with a lower end of a lower-pressure rectification column. Cold compressed air is separated into oxygen-rich and nitrogen-rich liquids in the higher-pressure column. Those liquids are transferred to the lower-pressure column for separation into nitrogen-rich and oxygen-rich products.
Large quantities of energy are required to compress the feed air in the air separation plant. The prior art teaches a number of techniques for energy savings in such installations. Coveney, in U.S. Pat. No. 3,731,495, discloses an energy reduction system which uses a nitrogen-quenched power turbine. A portion of the compressed feed air is mixed with fuel and combusted. The hot combustion mixture is then quenched with waste nitrogen-rich gas from the lower-pressure column and the resulting gaseous mixture is expanded in a power turbine. The expansion provides energy to compress the feed air to the system. The aforesaid Olszewski et al. U.S. Pat. No. 4,224,045 describes an improvement of the Coveney system wherein waste nitrogen is compressed prior to turbine expansion, thereby providing an increase in energy efficiency.
In an integrated gas turbine/air separation plant, the operation of the gas turbine takes precedence over that of the air separation unit. As is known to those skilled in the art, the gas turbine is employed to drive an electric generator which produces power that is distributed to users over a connected network. At times, the gas turbine may need to enter a turndown mode in response to a decrease in power demand. The gas turbine turndown mode may or may not correspond to a turndown operation of the air separation unit which is, in turn, related to the operation of the gasifier unit.
When a gas turbine is turned down, the operating pressure of the gas turbine is lowered, thereby causing a decrease in the pressure of the air feed from the turbine to the air separation unit. When the air separation unit air feed pressure is lowered, the operating pressure must be lowered and, as a result, the air separation unit produces less product. Further, the pressure levels of the oxygen and nitrogen products will be reduced at the exit of the air separation unit. When required product pressures at the use point are the same at turndown as they are at the design point of the system, the turndown operating point will exhibit significantly higher compression ratio requirements for the product compressors. This will result in higher capital cost for the system due to the increased requirements placed upon the product compressors.
To minimize the affect of gas turbine turndown on the operation of an air separation unit, the compression equipment and air separation unit could be designed to handle the higher pressure differentials expected under turndown conditions. However, this would require that the equipment be oversized for the design point and would result in inefficient operation. For instance, a gas turbine that is operating in a turndown mode may deliver the feed air to the air separation unit at a pressure that is 30 to 50 pounds per square inch absolute (psia) lower than the design point operating pressure. In order to deliver product gas from the air separation unit at a constant pressure over the possible operating range of the gas turbine, product compression equipment must be capable of compressing the product for the case with the highest pressure ratio--which is at the turndown operation. To the extent that that pressure ratio can be minimized, capital costs of the compression equipment will be reduced.
Accordingly, it is an object of the invention to provide an improved method and apparatus for operation of an integrated gas turbine/air separation plant wherein compression pressure ratios experienced under turndown conditions are reduced.
It is a further object of this invention to provide an integrated gas turbine/air separation plant wherein reduced turndown compression ratios are achieved by supplying lower purity oxygen to the use point.